1. Field of the Invention
This invention relates to thermographic materials. More particularly it relates to substantially colorless thermographic layers on a substrate. Many existing compositions exhibit a yellow or brown color cast which is caused by colored thermally sensitive metal compounds such as iron stearate. This invention uses compositions containing colorless iron salts which are thermally reactable to give a visible image.
In commercial applications, thermally developed labels are sought which not only provide visible images but which are also capable of being read by optical scanners using near infrared radiation (NIR). The images resulting from catechols with certain substituents exhibit low discrimination in the NIR. Other catechols give good discrimination both visually and to NIR.
2. Background of the Art
For many years heat-sensitive imaging sheets have been used for copying, thermal printing, thermal recording, and thermal labeling. Many of these materials involve thermally increasing the reactivity of two or more components of a color forming reaction which do not react at normal ambient temperatures. Reactivity is often enhanced by melting of one or both reactants which are physically separated from one anoher. Separation is accomplished either by dispersion in a single coated layer or by being situated in two different coated layers. Several general classes of color forming reactants have been used, of which two common ones are (a) leuco lactone or spiropyran compounds reactable with phenolic compounds (e.g. U.S. Pat. Nos. 3,829,401 and 3,846,153) and (b) heavy metal salts of organic acids reactable with ligands to give colored complexes (e.g. U.S. Pat. Nos. 2,663,654, 3,094,620, 3,293,055 and 3,953,659).
Commercial preference for the heavy metal salt class has often resulted from the high stability and near black color of the images produced (U.S. Pat. No. 4,531,141). Of the heavy metals used, iron, nickel, and cobalt are common and ferric iron appears to be preferred (U.S. Pat. Nos. 2,663,654, 3,953,659 and 4,531,141).
Two objections raised to the ferric salt-phenolic ligand systems are the colored nature of the unreacted ferric salt and the background stain often experienced because of the insidious, slow reaction of the two reactants on storage or during coating. Indeed, if the reactants are intimately mixed they often react rapidly at room temperature (U.S. Pat. Nos. 3,442,682, 4,531,141). The first objection has led to the use of white fillers (U.S. Pat. No. 4,531,141) or other incident light scattering devices (e.g., "blushing" the surface of the layer as in U.S. Pat. No. 3,953,659) to reduce the observed color tint of the coated layer. The second objection has led to the use of stabilizing compounds added to the reactive layer (U.S. Pat. Nos. 2,663,654, 3,442,682) and more particularly to the physical separation of the two reactants (U.S. Pat. No. 4,531,141) either by dispersion as separate micro-particles (U.S. Pat. Nos. 2,663,654, 3,094,620, 3,111,423, and 3,293,055) or by separating the reactants in distinct but adjacent layers (U.S. Pat. Nos. 3,111,423 and 3,442,682).
As indicated earlier, a considerable list of heavy metals has been used in their organic acid salt form to give thermographic images (U.S. Pat. Nos. 3,111,423 and 3,293,055). Some heavy metals giving colorless salts have been used and will be found amongst those listed in these two references. Such heavy metals (e.g., zinc), however, must be reacted with ligands which themselves contribute color to the image and may indeed have a color cast before reaction. These heavy metaIs have not successfuIIy provided satisfactory thermographic materials which are truly colorless and also give a deep near black color on thermal exposure.
Thus the art discussed so far shows consistent interest in two problems of ferric chelate imaging (a) the colored nature of ferric organic acid salts and (b) the difficulty in controlling the room temperature reactivity of such salts with the range of ligands available.
More recently there has been interest evinced in obtaining thermographically reactive iron salts which are colorless and which give sharp, high density images when reacted with a colorless ligand. Organo-phosphates of ferric iron are known in the art to be amongst the few colorless ferric salts (Smythe et al., J. Inorg. Nucl. Chem., 30 1553-1561, (1968)). In U.S. Pat. No. 4,533,930 it is disclosed that such organophosphates and the equivalent thiophosphates can react with a variety of ligands under the influence of heat to give colored results. Ferric salts of organophosphinic acid and organophosphonic acids are included. Some of these organophosphates and many of the thiophosphates have some color cast before reaction which appears to be obscured by the use of white filler in the thermosensitive compositions. In the thermographic materials disclosed, the reaction of the ferric salt with the ligand at ambient temperatures is precluded by either dispersing each reactant in microparticulate form in the binder or by providing separate but adjacent layers for the two reactants. These conditions are explicitly identified in the claims by the wording "said metal compound and said ligand compound being physically separated from one another . . . ". Also in this patent there are disclosed pressure-sensitive manifold papers in which at least one of the two reactants is encapsulated as a solvent solution. When the microcapsules are burst by pressure, the reactants come in contact and immediately react at room temperature to give a colored result. This patent further discloses the use of ferric organophosphates containing organic acid moieties as formed by the aqueous reaction of a ferric salt, an alkali metal organophosphate, and an alkali metal salt of an organic acid. These are disclosed as giving the initial material better "color forming properties" and giving better image colors (Column 5 lines 38-39) than the simple organophosphates. Excess organic acid salt is disclosed as degrading the white color. It is of significance that the inventors do not consider the choice of the ferric salt used in the preparation to be important. In fact they specifically mention ferric chloride and ferric sulfate (Column 6 lines 10-17) and all of their examples use ferric chloride.